Spread spectrum communication in its basic form is a method of taking a data signal that is used to modulate a sinusoidal carrier and then spreading its bandwidth to a much larger value, e.g. in a global positioning system (GPS) application by multiplying a single-frequency carrier by a high-rate binary (−1,1) pseudo-random noise (PRN) code sequence that is known to GPS users. Thus, the signal that is transmitted includes a data component, a PRN component, and a (sinusoidal) carrier component.
At the receiver, a synchronized replica of the transmitted PRN code is required to de-spread the data sequence. Initial synchronization, called acquisition, is followed by fine synchronization, which is called tracking.
The present invention relates to acquisition. Acquisition is the process by which the replica PRN code is synchronized (to within a small timing offset) with the code conveyed by the received signal either for the first time or after losing a previously acquired signal, and also by which the carrier frequency of the received signal is determined. Thus, to acquire a signal, an acquisition system must accurately determine any frequency-shifting of the received signal from the transmitted frequency in order to accurately wipeoff (remove) the carrier signal. Frequency-shifting can be caused by relative motion of the transmitter and receiver (Doppler-shifting) as well as by clock inaccuracies (so that a transmitter and receiver sometimes do not agree on what is in fact the same frequency). The carrier frequency-shifting results in a modulation of a code component after carrier wipe-off in the receiver. Thus, in acquiring a signal, it is also necessary that the replica code sequence be not only time-aligned with the received code sequence, but also modulated to compensate for the frequency-shifting so as to fully eliminate the PRN sequence and leave behind only the data conveyed by the received signal. The acquisition process is therefore a two-dimensional search, a search both in code phase and in frequency.
For low level signals additional frequency refining is sometimes necessary for closing tracking loops. Because of the two dimensional search needed for initial acquisition, a useful strategy is to first search with as coarse a frequency grid as possible in order to accelerate the search. As long as the replica code is aligned with the incoming signal in the code dimension one can refine the carrier frequency using only a one-dimensional but more fine resolution search in frequency.
A bottleneck here is the data modulation, i.e. the modulation of the signal by the data. In the initial acquisition stage, one strategy that is used is to take the frequency information from a coherent stage. The coherence length increase is limited due to 50 Hz navigation message and it reduces the gain of the system beyond 20 msec integration. Thus, for maximum gain, the resolution of coherent processing based methods is limited to 50 Hz. To go beyond this range one must either compensate for data modulation or sacrifice gain.
In weak signal conditions, a fine acquisition stage is required for tracking initialization. The frequency resolution of initial acquisition schemes is limited, due to the complexity of the two-dimensional search (i.e. for code phase and for any shift in the carrier frequency) and due to the data rate of 50 Hz (i.e. the navigation data rate for a GPS receiver). To obtain fine resolution, the data modulation must be wiped off, which according to the prior art has been accomplished either by bit detection, which is not always possible in weak signal conditions, or by a search over all possible data modulations of the signal as sampled in a set of samples, which imposes a significant computational burden. Another technique has been to use so-called zero-padding of a short segment of a signal so as to end up with a segment of a desired length, suitable for spectral analysis using a discrete Fourier transform (DFT) with a frequency spacing determined by the length of the zero-padded signal segment. Zero-padding with fine frequency spacing, without more however, does not yield fine frequency resolution in a noisy environment.
What is therefore needed, especially in weak signal conditions where sacrificing gain is sometimes not practical, is a computationally efficient way to more finely resolve the carrier frequency search than is possible without taking into account the data modulation.